The present invention relates to contact material employed for an electrically and mechanically sliding portion, and more specifically to the contact material and clad composition material containing the contact material which may be employed in a commutator equipped in a direct current motor, especially in a direct current compact motor, having a sufficiently long life and a low starting voltage when it is employed for the material of a compact disk spindle.
In recent years, the number of apparatuses in which sliding contact occurs has been increased in the field of electronic industries so that the development of new sliding contact material and the research regarding abrasion have been extensively conducted. The abrasion and the contact resistance are problematic in the sliding contact material, but this abrasion phenomenon is complex so that there are many aspects academically unelucidated.
Even though the metal surface of the contact material is intended to be finished relatively smoothly, the microscopic observation reveals that the surface is not a completely flat surface but ordinarily possesses minute unevenness. Although two metal surfaces appear to be in contact with each other with a large contact area, they are actually in contact with each other only with isolated projections due to the existence of the several concaves and convexes.
The abrasion due to the friction is basically proportional to the strength of a contact force and inversely proportional to the hardness. A temperature, a humidity, a corrosive component, organic vapor, dust and the like are factors introducing a change in the abrasion and electrical performances (contact resistance).
The wear in the sliding contact material is largely divided into adhesive wear and abrasion wear. The adhesive wear takes place by means of the welding between the metals of the actual contact potions or the projections so as to tear off the softer metal which is shifted to the harder metal.
The abrasion wear is induced when two groups of material having largely different hardnesses are rubbed with each other or when two groups of soft material one of which contains hard particles are rubbed.
The sliding contact material is widely utilized for an earth ring, a rotary switch and other devices inclusion a direct current compact motor for a compact disk spindle and a commutator employed in the direct current compact motor. Three-layered clad composite material employed as a conventional commutator for a compact disk spindle of a direct current compact motor is known to consist of a surface layer composed of an Au--Ag alloy consisting of 40% in weight of Ag and a balance of Au, an intermediate layer composed of an Ag--Cu--Ni alloy consisting of 4% in weight of Cu. 0.5% in weight of Ni and a balance of Ag and a base layer composed of a Cu--Sn--Ni alloy consisting of 9.5% in weight of Ni, 2.5% in weight of Sn and a balance of Cu.
Another pertinent prior reference discloses a rectifier having as a commutator an Ag--Pd--Cu alloy essentially consisting of 2 to 10% in weight of Cu, 2 to 10% weight of Pd and a balance of Ag. However, in this rectifier, the improvements on the performances, especially under a high temperature circumstance, such as the increase of a contact resistance and instabilities generated by black powder produced due to Pd and the generation of an electrical noise have been requested.
With the miniaturization of audio devices in the recent years, the direct current compact motor is equipped near a heating element such as an amplifier so that the temperature of the motor even at normal operation condition may reach to 70.degree. C. Especially when the motor is equipped in an automobile, the temperature under the midsummer burning sun may reach over 70.degree. C.
Generally, the life of the direct current compact motor equipped with a brush under the high temperature circumstance is unexpectedly short. While the above motor has a life of about 6000 hours under a circumstance of a temperature of 25.degree. C. and a humidity of 60%, its life may be reduced to about 200 hours under a circumstance of a temperature of 70.degree. C. and a humidity of 5%.
Accordingly, the development of a direct current compact motor which does not lose the durability under the high temperature has been demanded.
As a result of detailed investigation of the reasons why the life is reduced, it has been clarified that the commutator material is scraped off by means of a brush at the time of sliding between the commutator and the brush under a high temperature circumstance, which is deposited on the brush surface to form projection-like sediment, and the projection acts as if it were a blade to scrape off the commutator material long and narrow.
The long and narrow needle-like powder thus produced fills the spaces among the divided cylindrical commutator to short-circuit and electrically conduct the divided commutators so as to result in the stopping of the motor.
Even if the electrical conduction does not occur, the abrasion rate under the above high temperature condition is larger than that of the condition of a temperature of 25.degree. C. and a humidity of 60%, and the scraping-off of almost all the motors reaches to the Cu alloy of the base layer in 500 hours not only to increase the contact resistance but also to hinder the conduction by means of CuO derived from the exposed Cu so that the motor functionally stops.